Ultrahigh-frequency high-impedance passive voltage probe

Abstract

The most commonly used type of voltage probe for high-bandwidth applications is the high-impedance passive probe. It is reliable, rugged, simple to use, and inexpensive. However, because it has a relatively high input capacitance, it is not truly “high impedance” to the higher frequency components of a test signal. Consequently, connecting the probe can distort the signal under test (e.g., by increasing the rise and fall times of a fast pulse). Also, the probe input capacitance forms a resonant circuit with any parasitic inductance in the probe ground lead. In response to a short-duration step in voltage, this can cause spurious oscillations in the input signal to the instrument. Active probes generally have a much lower input capacitance, thereby minimizing these problems. Also, active probes have been able to achieve much higher bandwidths than those of high-impedance passive probes. But active probes are very expensive, and they are not as rugged or as reliable as passive probes. They also require a power source, and they can introduce a DC offset error to the measured signal. The present invention is for circuitry that greatly increases the bandwidth of high-impedance passive probes. The invention employs a plurality of separate circuit paths for the AC components of a test signal to bypass the attenuating resistor at the probe tip. This circuitry also increases the input impedance for the higher frequency components of a test signal. Thus the invention allows the design of high-impedance passive probes that are competitive in performance with active probes while retaining the much lower cost, and the other inherent advantages, of passive probes. Also disclosed, are voltage probes with selectable attenuation factors of specific values that combine user convenience with relatively high bandwidth.